as a mode of salutation, comes from its use to express reverence or worship. Thus to adore idols and to kiss idols mean the same thing. Indeed, the word adore signifies simply to carry the hand to the mouth, that is, to kiss it to the idol. We still kiss the hand in salutation. Various parts of the body are kissed to distinguish the character of the adoration paid. Thus, to kiss the lips is to adore the living breath of the person saluted; to kiss the feet or ground is to humble oneself in adoration; to kiss the garments is to express veneration to whatever belongs to or touches the person who wears them. “Kiss the Son, lest He be angry” (Ps. ii. 12), means Worship the Son of God. Pharaoh tells Joseph, “Thou shalt be over my house, and upon thy mouth shall all my people kiss,” meaning they shall reverence the commands of Joseph by kissing the roll on which his commands would be written. “Samuel poured oil on Saul, and kissed him,” to acknowledge subjection to God's anointed (1 Sam. x. 1). In the Hebrew state, this mode of expressing reverence arose from the form of government established, whether under the patriarchal or matrimonial figure.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Kiss from Fact Monster: